1. Field of the Invention
The present invention relates to a numerical control device for controlling a machine having an axis of rotation other than an axis of linear motion, and a numerical control method thereof.
2. Description of the Related Art
Machines having an axis of rotation other than an axis of linear motion are used as machine tools that work the free form surfaces of molds and the like. Five-axis machine tools equipped with two rotating axes and linearly moving axes X, Y, and Z are known. Because such machines are equipped with rotating axes, tools can be tilted to work in any direction with respect to the work surface of the workpiece in association with the rotation of these rotating axes. Because the amount of compensation to the tool length changes due to the tilting of the tool, work is performed while compensating for the tool length (Refer to Japanese Patent Application Laid-open Nos. H3-109606 and H5-100723, for example).
Known is an invention (Japanese Patent Application Laid-open No. 2001-269839) for corrects the spindle head error in a five-axis machine tool equipped with members that rotate about the linearly moving axes X, Y, and Z, with a C-axis, which is the axis of rotation about the vertical axis line, and with a B′-axis inclined at a prescribed angle with respect to the C-axis, by fixing the B′-axis at a prescribed angle; measuring the spindle end position for each prescribed swivel angle when rotating a member about the C-axis; performing a multiple regression analysis with a regression equation and computing the swivel plane from this measurement data; computing the axial vector of the C-axis member; fixing the C-axis at a prescribed angle; measuring the spindle end position at every prescribed swivel angle when swiveling a member about the B-axis; performing a multiple regression analysis with a regression equation, computing the swivel plane from this measurement data, and computing the axial vector of the B-axis member; computing the spindle end position in a state in which the members swivel about the axis lines of these axial vectors; taking the difference between this spindle end position and the precise spindle position as the spindle head position error; inverting the sign of the spindle head position error and using the result as the correction value; and offsetting the coordinate system of an NC program in order to correct the tilt error from the vertical axis line of the C-axis, the incline angle error of the B′-axis, and the error in the position of the center of rotation of the B′- and C-axes.
The tool length correction methods cited in the above-mentioned Japanese Patent Application Laid-open Nos. H3-109606 and H5-100723 do not take the incline and misalignment of the center of an axis of rotation into consideration. Cases in which the center of an axis of rotation diverges or becomes inclined from the original position (cases in which the control system for executing a work program has an error between the recognized center axis of rotation and the direction thereof, and the actual center axis of rotation and the direction thereof) are not taken into consideration. Also not taken into consideration is the case in which the center of swiveling about the main axis diverges or becomes inclined from the original position.
However, it is difficult to accurately manufacture the center of an axis of rotation in the original position and direction, and the turning center of the spindle in the original position and direction because of factors related to the manufacture of machine tools, and errors occur. In the above-described Japanese Patent Application Laid-open No. 2001-269839, the center position error of the axis of rotation and the tilt error are taken into consideration, and the error of the spindle head position is corrected, but with this method, one of two axes of rotation is fixed, a multiple regression analysis is performed with a regression equation on the basis of the data obtained from measuring the spindle end position for each position in which the other axis is rotated at a prescribed angle, the swivel plane is computed, the axial vector is computed, the spindle end position is computed in a state in which these axial vectors are swiveled; the difference with the original spindle position is taken to be the spindle head position error; and the NC coordinate system is offset in accordance with the spindle head position error, resulting in complicated processing.
The method cited in the Japanese Patent Application Laid-open No. 2001-269839 has the following drawbacks.
For each positioning of the two axes of rotation, these axes are swiveled a prescribed angle and measured each time, and the actual direction vectors of the axes of rotation thereof are computed. Therefore, this approach is disadvantageous in that time is required for measurement each time the axes of rotation are positioned, and that the invention cannot be used in continuous working that includes movement of the axes of rotation. Assuming that the centers of rotation of the two axes of rotation intersect with each other, no consideration is given to the case when there is divergence between the two centers of rotation. Another drawback is that no consideration is given to the error between the center of rotation of the spindle and the axis of rotation.